Operando X-ray diffraction and X-ray absorption studies of the structural transformation upon cycling excess Li layered oxide Li[Li1/18Co1/6Ni1/3Mn4/9]O2 in Li ion batteries

Abstract

Operando X-ray diffraction and X-ray absorption spectroscopic (XAS) measurements were carried out on layered Li[Li_1/18Co_1/6Ni_1/3Mn_4/9]O₂ to investigate the structural changes during the first charging and discharging. The XRD results showed a phase transformation from rhombohedral to monoclinic during the charging process. X-ray absorption near-edge spectroscopy (XANES) measurements showed that the major charge compensation takes place at the Ni metal site as occurring through a two-step process, i.e. Ni²⁺ → Ni³⁺ → Ni⁴⁺, while the cobalt ions (Co³⁺) and manganese ions (Mn⁴⁺) remain unchanged, during Li-ion insertion/de-insertion. Extended X-ray absorption fine structure (EXAFS) results at the Ni edge showed a significant distortion of the Ni–O and Mn–O shells, while no significant distortion was observed at the Co–O shell during charging and discharging. From the structural analysis results, the cobalt doped Li[Li_1/18Co_1/6Ni_1/3Mn_4/9]O₂ cathode was shown to undergo a partial increase of oxygen ions at the Mn ion environment due to the oxygen ion migration from the bulk to the surface of the electrode structure during charging. It indicated that the LiMn₂O₃ domain plays an important role in the oxygen-activation plateau. The mechanism also showed that the partial amount of Co in a transition metal slab of excess lithium layered oxide materials can stably maintain the lithium ions in the transition metal inter slabs and the surface electronic structure of oxygen ions is reversible during electrochemical cycling between 4.6 V and 2.5 V.